This invention relates generally to an electrophotographic printing machine, and more particularly concerns a sheet transport for moving a sheet in a path to enable a toner image to be transferred thereto. The invention also particularly concerns a sheet transport for moving a sheet in a recirculating path to enable successive toner powder images to be transferred thereto in superimposed registration with one another while minimizing unwanted belt movement that might otherwise adversely affect image quality.
The marking engine of an electronic reprographic printing system is frequently an electrophotographic printing machine. In such a machine, a photoconductive belt is charged to a substantially uniform potential to sensitize the belt surface. The charged portion of the belt is thereafter selectively exposed. Exposure of the charged photoconductive belt or member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
In the process of black and white printing, the copy sheet is advanced from an input tray to a path internal the electrophotographic printing machine where a toner image is transferred thereto and then to an output catch tray for subsequent removal therefrom by the machine operator. In the process of multi-color printing, the copy sheet moves from an input tray through a recirculating path internal the printing machine where a plurality of toner images is transferred thereto and then to an output catch tray for subsequent removal. With regard to multi-color printing, a sheet gripper secured to a transport receives the copy sheet and transports it in a recirculating path enabling the plurality of different color images to be transferred thereto. The sheet gripper grips one edge of the copy sheet and moves the sheet in a recirculating path so that accurate multi-pass color registration is achieved. In this way, magenta, cyan, yellow, and black toner images are transferred to the copy sheet in registration with one another.
Some systems which have been designed for transporting a copy sheet into registration with a toner image developed on a moving member accelerate the copy sheet during transfer of the toner image from the moving member to the copy sheet. Such acceleration may occur when the leading portion of the sheet is being negotiated through a nonlinear path while at the same time the trailing portion of the copy sheet is traveling through the transfer zone. The above acceleration may cause a deterioration of the integrity of the image produced on the copy sheet due to slip between the copy sheet and the moving member while the sheet is traveling through the transfer zone. An example of the above deterioration is a blurred or smeared image produced on the copy sheet.
A problem that confronts machines designed for color copying, which does not necessarily occur in black and white copying, is unwanted belt motion in both process and lateral directions during the movement of the photoreceptor belt. Unlike black and white copying, in a xerographic color copier, using a multiple pass color registration scheme, the accuracy of the color on color alignment or registration is extremely important to image quality. Where excessive lateral belt motion occurs, the images are not properly registered to create an acceptable image.
For acceptable images, lateral belt motion should be limited to about 80 microns or less. It has been found that lateral motion from pass to pass on photoreceptor belt systems could exceed 300 microns or more. After incorporating the invention described herein, this lateral belt motion can be less than 80 microns, and certainly less than the 100 microns which is desirable.
The invention described herein utilizes three Low Lateral Force (LLF) rolls supporting the photoreceptor belt to minimize lateral belt motion on a multi-pass color copier. A combination of the rolls, the module mounting, belt guidance and tolerance system all contribute to achieving this desirable reduction in lateral belt motion. Although any one of these features can be included independently of the other, it is found that all contribute to minimizing undesirable belt motion.
In the past the primary function of LLF rolls has been to allow the reaction force transmitted into the belt edge to be dissipated in deflecting LLF pedals. This provides edge guidance of a flexible belt without edge damage, and has been used in many production AMAT belt products. However, the multi-pass color on color registration system requires that the belt move in as slow, lateral rate as possible to minimize color to color placement errors. Since lateral motion in a dynamic system cannot be completely eliminated, it is necessary to minimize the rate at which it changes in time. The system described above, referred to as a "stiff" LLF system, achieves this result. It has been found that the lower the axial roll stiffness, the higher the axial belt motion. Where stiffer rolls are used, the less capable the rolls are of dissipating the edge force from the edge guide.
The system developed herein is one that minimizes lateral belt motion, as it is edge guided, through the use of axial roll stiffness to reduce the lateral belt tracking rate and thus the amount of rebound reaction to the edge guide force. For this purpose the drive and strip rolls have the highest axial roll stiffness to provide primary resistance to lateral motion. The tension roll requires lower axial roll stiffness in order to do the majority of the dissipation of the belt edge force as the belt is guided at this roll.
The alignment control to enable running such a stiff low lateral force is accomplished by basically three approaches. A single piece, widely spaced, mounting through shaft is utilized to reduce the tolerance impacts of the xerographic module drawer inaccuracies and reduce the inboard to outboard misalignment. An integral tension slide/side plate guidance control system is employed to reduce the misalignment of the tension roll, which is the largest contributor to internal belt module misalignment. Finally, controlling the tolerances of the mounting system for the xerographic module drawer, the tolerance control plan assures that the other elements of the system are mounted properly in the machine frame to achieve the desired tolerances and resulting low lateral belt motion.
The above has been a brief description of deficiencies in the prior art and advantages of the invention. Other advantages will become apparent from the detailed description of the preferred embodiment which follows.
Other features of the present invention will become apparent as the following description proceeds and upon reference to the drawings.